1. Field of the Invention
The present invention relates to an adaptive diversity apparatus for digital communication.
2. Description of the Related Art
A conventional adaptive diversity apparatus will be described with reference to FIG. 1 and FIG. 2. FIG. 1 is a block diagram of the conventional adaptive diversity apparatus. FIG. 2 is a radiation pattern view of assistance in explaining reception (transmission) radiation pattern. In FIG. 1, complex signals which are received by reception antennas 1, 2, 3 and 4 at a time t, A/D converted and quasi-coherent detected complex signals are designated S.sub.1 (t), S.sub.2 (t), S.sub.3 (t) and S.sub.4 (t), respectively.
A reception radiation pattern controller 9 transmits complex weights W.sub.1 (t), W.sub.2 (t), W.sub.3 (t) and W.sub.4 (t) to multipliers 5, 6, 7 and 8. The multipliers 5, 6, 7 and 8 multiply the abovementioned complex signals by the complex weights. The multiplied complex signals are added by an adder 12. At this time, when the output of the reception radiation pattern controller 9 is S(t), the output S(t) can be expressed as the following formula (1): EQU S(t)=.SIGMA..sub.1.sup.4 W.sub.i (t)S.sub.i (t) (1)
By thus composing the reception signals from a plurality of antennas after multiplying them by appropriate complex numbers, it is possible to provide all the antennas with radiation pattern on a plane as shown in FIG. 2. For example, in FIG. 2, when a desired signal is coming from the direction of the arrow 21 and an interference signal is coming from the direction of the arrow 22, the reception radiation pattern controller 9 controls the radiation pattern as shown at the reference numeral 23 of FIG. 2. That is, the controller 9 controls the radiation pattern so that the desired signal is received with high intensity and the interference signal is received with low intensity. Thereby, the reception performance can be improved.
A decider 11 outputs the result D(t) of hard decision of the composite signal S(t). An error detector 10 outputs the difference S(t)-D(t) between the composite signal S(t) and the detected result D(t). The reception radiation pattern controller 9 updates its output complex weights W.sub.1 (t), W.sub.2 (t), W.sub.3 (t) and W.sub.4 (t) based on the output of the error detector and S.sub.1 (t), S.sub.2 (t), S.sub.3 (t) and S.sub.4 (1).
For example, when the reception signal vector is expressed as Sig(t)=(S.sub.1 (t), S.sub.2 (t), S.sub.3 (t), S.sub.4 (t)).sup.T and the output of the reception radiation pattern controller, as W(t)=(W.sub.1 (t), W.sub.2 (t), W.sub.3 (t), W.sub.4 (t)).sup.T, the updated complex weights can be expressed as the following formula (2): EQU W(t+1)=W(t)+.mu.(S(t)-D(t)).sup.T Sig(t) (2)
Here, .mu. is a step factor.
In the above-described conventional adaptive diversity apparatus, by successively updating the weights which are multiplied by the input signals from the antennas, even when the transmitting side moves, the radiation pattern can be formed in accordance with the movement.
In the conventional apparatus, when the movement speed of the transmitting side is low, the radiation pattern can be formed in accordance with the movement; however, when the movement speed of the transmitting side is high, since the complex weights do not converge, the processing by the reception radiation pattern controller cannot respond to the movement, so that a correct radiation pattern cannot be formed.